The OQPSK modulation method has the desirable attributes of providing a constant amplitude envelope at a narrower frequency spectrum than the more conventional, non-offset QPSK. These desirable attributes enable an OQPSK transmitter to drive a simple, high efficient carrier amplifier and derive a suppressed carrier wave having minimal interference from transmissions of other nearby radio frequency channels. A coherent demodulator is, however, required and must determine and acquire the frequency and phase (within four-fold ambiquity limitations) of the missing, i.e., suppressed, carrier and achieve data bit frequency and phase synchronization. Often it is required to perform these tasks in a minimum time period, under conditions of high bit error rate (BER) or poor signal-to-noise ratio (SNR), with relatively large frequency uncertainties. These three requirements are in conflict, however.
The typical receiver approach to carrier "recovery" and bit synchronization involves the use of first and second servo loops respectively called a carrier tracking loop (CTL) and a bit synchronizer loop. Both loops are often designed as phase locked loops (PLL's). For high bit error rate (BER) performance, it is necessary to generate phase-stable carrier and bit references even under conditions of poor BER. Therefore, both loops are required to have relatively narrow bandwidths, a factor that limits (1) the amount of frequency uncertainty that can be accommodated without frequency search and (2) the acquisition rate, with or without frequency search.
The present invention therefore addresses (1) preamble signal structure, (2) the receiver loops and (3) an acquisition method and apparatus for reliable and fast signal acquisition under the above conditions for OQPSK modulated signals.
There is a problem associated with all CTL circuitry for suppressed carrier demodulation where the carrier frequency uncertainty is a significant fraction of the bit rate. The problem involves the possibility of the loop temporarily locking or attempting to lock onto modulation sidebands, instead of the proper carrier frequency. A more serious problem can exist when the OQPSK modulation form is equivalent to minimum shift keying (MS) as discussed in the copending, commonly assigned application "QPSK Suppressed Carrier With Rotating Reference Phase", Ser. No. 30,905, filed Apr. 17, 1979. In this equivalent situation with half-cosine shaping of the envelope in each of the quadrature signal components, it has been found that for the duration of a certain short-term data pattern, steady single-frequency sideband transmission can result. This sideband frequency can be offset in either direction from the carrier frequency, with the offset value being one-fourth the bit rate frequency.
A final problem involves the nature of an optimum-performing decision-directed control tracking loop (DDCTL). The DDCTL integrates information during a data bit interval and makes a polarity decision before applying the result as a correction signal to a voltage controlled reference oscillator of the DDCTL. Thus, in the ideal at least, bit synchronization is required before the CTL can function properly, but bit synchronization is not normally possible until the CTL is locked. This dichotomy suggests and, almost makes mandatory, the often-used concept of a special message preamble enabling the orderly acquisition and operation of the CTL and bit synchronizer before the message proper is transmitted.